Periodically, a nuclear well logging circuit must be calibrated. Calibration involves testing with a standard. The output signal is observed and compared with an expected or anticipated output signal to determine whether or not the circuitry in the nuclear well logging apparatus is operating correctly.
Sometimes, such tools are suspected to malfunction in which instance it is necessary to do trouble shooting to locate a prospective difficulty. One difficulty in trouble shooting is the necessity of disassambly; tool assembly and disassembly before and after repair takes time. It is difficult to trouble shoot with a nuclear source because nuclear sources are, by nature, inexact, nonlinear or random in their output. By the very nature of the phenomena under investigation, the randomness poses a problem in knowing precisely what the source is doing so that the calibration can be carried out.
There is some difficulty in rigging a nuclear well logging tool for maintenance including both calibration or trouble shooting. The tool must be placed adjacent to some type of radiation source. Radiation sources for all types of tools would be necessary to service a tool in field circumstances. Some radiation sources cannot be readily shutdown, easily stored, or placed in a safe container. Rather, nuclear radiation sources used for calibration purposes must be treated as hazardous material and suitable precautions taken in light of this fact. Therefore, the present apparatus overcomes these handicaps to calibration or trouble shooting in that it furnishes a signal source for testing nuclear well logging tools without the impediment of safely hauling radiation sources. This apparatus furnishes a series of pulse of controlled pulse height distribution to enable calibration or trouble shooting of nuclear well logging tools.
The present apparatus can be used with a variety of nuclear logging tools. For instance, one type of nuclear logging tool is an apparatus which detects only pulses exceeding a certain height. Pulses below that height are discarded. Pulses above that height are counted. The circuit of this disclosure is an apparatus which provides pulses in a range of heights so that the trip point of the discriminator can be tested to thereby ascertain the level at which pulse detection occurs. This test circuit can be used with that type of instrument.
Another type of downhole nuclear logging tool utilizes multiple window analyzers. A number of pulses are received over a range of pulse heights. There might be two or more pulse height windows in the circuitry. This apparatus furnishes pulses to enable the pulse windows to be triggered selectively at different levels to ascertain the pulse height range to each window in the circuitry. Pulses within each window are thus summed or counted by this circuitry
Another type of nuclear logging tool incorporates a spectroscopic detector. Inevitably, this involves conversion of an analog signal into a digital signal, typically being accomplished in an analog to digital converter. Through the use of the test instrument of this disclosure the linearity of the converter can be verified.